1. Field of the Invention
The present invention relates to battery systems and, more specifically, to circuits that limit current in battery systems.
2. Description of the Prior Art
Battery packs, such as those used to power cellular telephones and other portable electronic equipment, usually include devices to prevent substantial current flow from the cells when a short across a battery pack's terminals occurs. Such a device usually takes the form of a fuse, which opens when current flow exceeds a predefined threshold. Because a fuse opens permanently, the battery pack becomes unusable once an over-current situation occurs.
Many shorts are "soft" shorts that occur when the battery pack is disconnected from its host (which could be a piece of equipment or a battery charger). For example, a battery pack that is placed in a purse or a pocket may become shorted by a key or a gum wrapper. Since such a short is only transitory, it is desirable to have a battery pack that limits current during a soft short, but that is still usable once the battery pack is reconnected to its host.
Some battery packs include resettable circuits for interrupting the discharge current. An integrated circuit detects the voltage drop across a series resistor or across the on-resistance of a series field effect transistor (FET) and opens the FET to prevent any additional current flow. Additionally, a temperature and current activated polymer device such as a poly-switch may be used to interrupt high current flow. Unlike a fuse, in both of these cases the devices are resettable. However, in both cases a relatively high discharge current is required to activate the short circuit protection, but no protection is offered for a soft short where the discharge current may be significantly above the operating current of the host device but below the threshold of the hard short protection circuit. Additionally the accuracy of the over current threshold on such prior art devices is typically poorly and loosely defined
Simply opening a series discharge switch to prevent any current flow when the battery pack is disconnected from the host is not a viable option in systems where the host requires power on demand and where the host uses this power to indicate to the battery that it is attached to a host. If the battery simply removes power from its term anytime it is disconnected from the host, then the host will not know when to reapply power because the host never receives power to turn itself on and indicate to the battery that a host is reattached.
Therefore, there is a need for a current limiting circuit that restricts current flow from a battery when the battery is disconnected from a host, but that allows regular current flow from the battery when the battery is reconnected to the host.